Magnetoresistive elements and devices



Nov. 5, 1963 H. E. KALLMANN MAGNEJTORESISTIVE ELEMENTS AND-DEVICES 2 Sheets-Sheet 1 Filed Oct. 4, 1957 lNl/ENTO/P HE/NZ E KALLMANN BVMW AGENT Nov. 5, 1963 KALLMANN 3,109,985

MAGNETORESISTIVE ELEMENTS AND DEVICES Filed Oct. 4, 1 5 2 Sheets-Sheet 2 FIG. 7

38 34 32 l8 2 p I L g l P AMP lNVENTOl? HE/NZ E KALLMANN BVW AGE/VT United States Patent 3,109,985 MAGNETORESISTIVE ELEMENTS AND DEVICES Heinz 1E. Kalimann, New York, N.Y., assignor to Guiton Industries, Inc, Metuchen, NJ., a corporation of New Jersey Filed Oct. 4, 1957, Ser. No. 688,239 Claims. (til. 323-94) This invention relates to magnetoresistive elements, and to devices and apparatus comprising such elements as components.

The change in electrical resistance under the influence of a magnetic field is known as magnetoresistance. This effect has long been observed in the prior art as characterizing the behavior of a number of materials, such as bismuth. However, recent studies have disclosed that certain semi-conductor materials, such as for example indium antimonide and indium arsenide, particularly when doped with n-type impurities, are magnetoresistive to a much greater degree than previously known substances.

E. H. Hall discovered, in 1879, that when a certain type of conductor is placed in a magnetic field that is perpendicular to the direction of current in the conductor, an electrical potential appears in a direction perpendicular to the direction of the current, and to that of the applied magnetic field. It has been found that if the Hall voltage is short-circuited by external wire connections, the flow of Hall current enhances the increase of the observed resistance offered to the primary current in the conductor.

Prior-art elements comprising magnetoresistive materials, which have been designed to utilize the Hall current as a means for increasing the magnetoresistive efiect include a thin annular disk having electrodes on the inner and outer bounding surfaces, which was invented some 40 years ago by O. M. Corbino, While such an element shows a greater magnetoresis-tive effect than elements of rectangular form, it nevertheless has certain disadvantages. For example there is no linear relationship between the resistance of a Corbino disk and its displacement with respect to an inhomogeneous magnetic field. Moreover, an element of such form tends to have an in- .conveniently low resistance, since of necessity, the distance between the concentric electrodes is small compared to the width of the current path.

It is the general object of the present invention to improve the efficiencies of magnetoresistive elements and devices by making the greatest possible use of the aiding effect of the Hall current.

A more specific object of the invention is to provide elements characterized by a substantial increase in the apparent magnetoresistive effect and yet more convenient values of resistance than attainable with prior-art elements.

A further object of the invention is to provide a substantially linear relationship between the displacement of a magnetoresistive element with respect to a stationary magnetic field and the resultant changes in the resistance of the element.

In accordance with the present invention, these and other objects are realized in a combination which utilize magnctoresistive elements have a hollow cylindrical, or tubular, shape. In a preferred embodiment of the invention, the magnetic field is applied in a radial direction to the said elements.

A particular embodiment of the invention comprises a tubular magentoresistive element of the form described in the foregoing paragraph, mounted as a movable tube or sleeve in an annular gap formed by the concentric pole pieces of a permanent magnet, which has a cylindrical outer member concentric with a mushroom-shaped inner member. An intense, radial magnetic field is generated in the annual gap between the soft-iron pole pieces of the magnet, between which the tubular magnetoresistive element is movable axially, thereby producing a substantial variation in the resistance of the aforesaid tubular element.

In accordance with one form of the present invention, the external circuit need not be connected directly to the movable magnetoresistive element, but may instead of indirectly coupled by means of a movable wire loop through a stationary toroidal transformer core. In accordance with a modified form the magnetoresistive element is doubled back on itself to form two concentric tubes joined at one end and having both terminals at the other end, which two tubes move in and out of the radial field, and may readily be coupled to the external wire connections.

It will be apparent to those skilled in the art that a configuration including a cylindrical or tubular magnetoresistive element in accordance with the present invention is readily adapted for numerous applications, such as, for example, to indicate by their changes in resistance the variations in strength of a magnetic field, or alternatively, to indicate their own position when moved relative to an unvarying stationary magnetic field. The teachings of the present invention are also applicable to other types of devices, such as, for example, a microphone, in which the magnetoresistive tube or cylinder is moved with reference to the magnetic field by means of a diaphragm to which it is rigidly connected.

Other objects, features, and advantages of the present invention will be apparent to those skilled in the art, upon studying the detailed specification hereinafter, with reference to the attached drawings, in which:

FIGURES 1, 2, and 3 show, in perspective, several prior-art types of magnetores'istive elements;

FIGURE 4 shows, in perspective, a tubular magnetoresistive element formed in accordance with the teachings of the present invention;

FIGURE 5A shows, in perspective, a magnetoresistive element in accordance with FIGURE 4, in cooperating relation with a coaxially shaped permanent magnet, which provides a radial magnetic field;

FIGURE 5B is a front elevational showing of FIG- URE 5A.

FIGURE 6 shows a combination in which the magnetoresistive element in the arrangement of FIGURES 5A and 5B is connected to a single movable wire turn which moves in a slot in the center pole-piece of the magnet, and forms the primary winding of a torodial transformer;

FIGURE 7 shows a modification of the embodiment of FIGURE 6 in which the magnetoresistive element comprises two concentric tubes joined together at one end to form an annular sleeve which readily moves in the annular gap of the magnet, and which is readily coupled by a single turn to the transformer core;

FIGURE 8 is a schematic circuit diagram of the toroidal transformer of FIGURES 6 and 7;

FIGURE 9 shows a microphone in which the teachings of the present invention are applied.

Referring to the drawings, FIGURE 1 shows a priorart structure which takes the form of a parallelepipedon of magnetoresistive material, to two opposi e ends of which have been applied electrode coatings, connected to an external source of electric potential. If this oom bination is placed in a magnetic field which is per-pen dicular to the direction of the applied electrical potential, electrons travelling the length of the parallelepipedon are transversely deflected in a direction perpendicular to both the electrical and the magnetic fields, thereby increasing the resistance to current flow between the electrodes in accordance with the strength of the magnetic field. Moreover, as noted in the earlier part of the specification if the Hall potential, which exists between any two laterally spaced points X and Y on the magnetoresistive conductor, is short-circuited by an external wire connection not shown, a still further increase takes place in the resistance between the electrodes.

As a simple solution to the problem of short-cireuiting the Hall current, the prior-art structure of PEGURE 2 utilizes a thin rectangular element, having electrodes on its major parallel faces. in such a configuration, the electrodes themselves function as short-circuits, so that the percentage change of electrical resistance between the electrodes for a given magnetic field is substantially larger than in the structure of FIGURE 1.

However, the structure of FEGURE 2 also has certain disadvantages, in that, for instance, the dimension of the element parallel to the magnetic field is large, necessitating a relatively long magnetic gap; and further, the resistance between the electrodes is relatively very small, and inconvenient for most practical circuit applications.

Another prior-art configuration for 'inagnetoresistive bodies which was invented by O. M. Corbino, is a disk having a central circular hole. In this arrangement the electrodes are applied to the inner and outer curved surfaces. The merit of this arrangement is that the Hall current, at right angles to both the electric and the magnetic fields, readily flows in a circumferential direction without need for an external short-circuiting connection. Of all of the magnetoresistive devices known in prior art, this configuration offers the greatest change in resistance for a given change in magnetic field.

However, this configuration, also, has certain disadvanages, in that when it is moved with reference to the edge of a magnetic field, the change in resistance is not linear. Moreover, it presents a relatively low resistance between electrodes.

In accordance with the present invention, a cylindrical sleeve of magnetoresistive material, of the general form indicated in FIGURE 4 of the drawings combines all the desirable features of the prior art configurations, While avoiding the stated. difficulties.

Making specific reference to FIGURE 4, the cylindrical sleeve It? may comprise any material which is highly magnetoresistive. In the present illustrative embodiments, an ingot of indium antimonide is utilized which has been zone-refined by methods well known in the art to a purity of one part in a billion; and into which have been introduced n-type impurities, such as, for example ions of tellurium of sulphur in a concentration of about 50 parts per million. For the purposes of the present invention optimum impurity concentrations may be varied over the range through 10 n-type impurities per cubic centimeter, in accordance with the requirements of different specific applications, as discussed in detail in an article by R. K. Willardson and A. C. Beer entitled Magnetoresistance, New Tool for Electrical Control Circuits, Electrical Manufacturing, volume 57, Number 1, January 1956, pages 79 through 84. Moreover, a group of other factors of interest in the design of a magnetoresistive device include the temperature co-efilcient of resistance, and the resistivity Within the working temperature range.

Indium antimonide of the purity and doping indicated may be obtained commercially from the Ohio Semi- Conductor Company of Columbus, Ohio.

For the embodiment under discussion, a polycrystalline cube about a quarter-inch in cross-section, which has been cut from an ingot of indium antimonide of the composition and purity indicated, and which is characterized by a resistivity within the range of 10- to 10- ohmcentirneter, and electron mobilities of about 75,000 centimeters per second per volt per centimeter, is placed in a cup consisting of specto-graphically pure graphite, and heated to a temperature of about 600 degrees centigrade, which is slightly above the melting point of the material in an inert atmosphere. In the present illustrative embodiment, the graphite cup has an inner diameter of about A of an inch, to conform to the desired outer diameter of the cylinder. A cylindrical pin, also of spectographioally pure graphite, having a diameter about 40 mils less than the inner diameter of the graphite cup is forced into the cup in a position concentric with the cylindrical cavity, forcing the soft or liquid indium antimonide between the two pairs of cylindrical walls. Heating of the assemblage is carried out in a graphite crucible which fits into the muffle of the furnace. As soon as the assemblage reaches the desired temperature, the furnace is turned off, care being taken to limit the temperature so as to reduce the degree of oxidation which takes place.

The assemblage is cooled slowly in the furnace; to room temperature, over a period of about two hours, to solidify the indium antimonide.

After solidification of the soft or liquid material, the external graphite mold comes apart for removal, the central graphite pin being removed by drilling. The resulting hollow cylinder of indium anti-monide is then subjected to grinding and burnishing to remove any remaining portions of the graphite, or oxidized portions. This may involve removing a layer anywhere from a few angstroms to a mil in thickness.

The finished tube or cylinder is about 4 inch long and has an overall diameter of about A inch, the curved wall being about 20' mils thick.

In the present illustrative embodiment, annular electrodes 12 and 14, each comprises a conductive layer of lead-tin solder, or silver deposited by any of the processes well known in the art, such as, for example, evaporation, on the flat ends of the tube or hollow cylinder 10. Conducting leads 16 and 18 are soldered to the respective electrodes.

Although certain definite dimensions have been given by way of illustration, it will be understood that the cylindrical sleeve or tube 10 may be as thin-walled, and as long and slim as convenient, permitting a relatively large resistance between electrodes.

Accordingly, if a source of electrical potential is connected across electrodes 12 and 14, producing an electrical field in longitudinal direction in the tube 10, and if a magnetic field is applied radially across the thickness of the tube 10, the Hall current flows around the tube perpendicular to both the longitudinal electrical field and the radial magnetic field.

FIGURES 5A and 5B of the drawings respectively show, in perspective and in front elevation, a system in which a magnet of coaxial form provides an intense radial field which is applied across the thickness of a magnetoresistive sleeve of the form described hereinbefore with specific reference to FIGURE 4.

Referring in detail to FIGURE 5A and 5B, the body of the coaxial magnet 27 comprises a tubular outer portion 24, having an outer diameter of, for example an inch, and an inner diameter of inch, and extends about /2 inch along the principal axis. This may comprise any of the permanent magnetic materials known in the art which are characterized by a relatively high maximum energy product. For the purposes of the present illustrative embodiment, the element 24 is composed essentially of a material known by the trade name Alnico 5, an alloy containing about 24 percent cobalt, 14 percent nickel, 8 percent aluminum, 3 percent copper, and the balance substantially iron; which alloy is characterized by a coercive force of 550 oersteds, a retentivity of 12,500 gauss, and a maximum energy product of 4.5 x 10 One end of the tubular portion 24 is closed by a circular soft-iron plate 26, about an inch thick, and having fixed to its center a soft-iron rod 26a which extends inwardly about /2 inch, and terminates in a head 28 about inch in diameter. The tubular portion 24 has a flange in the form of an annular soft-iron pole-piece 3t fixed to its other end, having an inner diameter of about inch and a thickness which matches that of the head 28, thereby forming between the two a narrow, annular gap 22 having a width which is a few mils greater than that of the element 110, in order to accommodate the latter closely, but without physical contact, and having an axial extent of not less than that of the said element, which in the case of the present illustrative embodiment is A inch long. Across this gap is impressed an intense magnetic field having a flux-density of the order of 1000 to 10,000 gauss.

The cylindrical sleeve or tubular magnetoresistive element is concentrically disposed between the inner and outer pole pieces 28 and 30 of the permanent magnet 27, so that the sleeve is movaJble in a-direction perpendicular to the intense magnetic field across the annular gap 22. The ends of the tube '10 are connected to a circuit which includes a conventional voltage source 29 of, for example, a few volts and a current indicating or utilizing means 31 Which may, for example, be a conventional ammeter.

In accordance with the arrangement of FIGURES 5A, and 5B, the tubular magnetoresistive element It) is movable with respect to the stationary magnet 27. Hence, the element 10 is shown partly outside of the magnetic field between pole pieces 28 and 30'.

The tubular element 10 may thus be thought of as comprising three parts connected in series: one part wholly Within the strong magnetic field, a part intermediate in the transitional fringe field, and a third part wholly outside it.

The total length of the first and third parts is constant, but their relative length varies with position, the length of the second part and the magnetic field it meets, remain unchanged. Assuming that the magnetic field gap 22 is of constant width, as shown, it will be evident that the element may be moved longitudinally into it either until the inner electrode 14 reaches the edge of the inner fringe field, or until the outer electrode 12 reaches the edge of the outer fringe field, the change of the total resistance being substantially linear with this displacement. To put it diiferently, the sole condition of such linear relationship is that both inner electrode end 14 and outer electrode end 12 of the element 110 can travel in regions of unvarying magnetic field strength, the one high, the other low.

In order to permit motion of the element 10 relative to the magnet 27, the leads I6 and 18 must be flexible. However, they must nevertheless have very low resistance, since even with the tubular shape, the resistance of the element 10 may be of the order of only a few milliohms. Accordingly, the leads 16 and 18 are of necessity relatively heavy and thick, comprising, for example, 18 gauge stranded copper wire. Such a device is not particularly suited for use in low inertia, low friction position-indica ing instruments.

I, therefore, prefer to substitute an electrical coupling arrangement which differs from that of FIGURE 5A and 5B in the manner shown in FIGURE 6, which is particularly adapted for such applications. This arrangement relies on an inductive transformer coupling between the movable magnetoresistive element 10, and the stationary external circuit.

'Referring in detail to FIGURE 6, elements are shown with designations which conform to those used on FIG- URES 4, 5A and 5B. The magnetoresistive element 10, the magnet 27, with central core 26a, tu bular outer element 24, and the soft-iron flanged pole pieces '28 and 30, which provide the annular gap 2 2, are the same as described with reference to FIGURES 5A and 53, except that the circular inner pole piece 28 contains a small notch 36, having a cross-section a few mils larger than the diameter of the wire extending through the thickness of the circular pole piece 28, which notch serves to accommodate the wire loop formed by the joined lead wires 16-18, attached to electrodes :12 and 14, respectively, of

element 10. Accordingly, as the magnetoresistive element 10 moves in and out, the accompanying wire loop moves smoothly in the notch 36. The loop 1648 forms a single primary turn, which is threaded thru the torodial core 32. The latter is a high-permeability core, comprising a material such as, for example, permalloy, upon which is wound a large number of turns terminating in leads 38 and 40' which are connected to an alternating voltage source 29 and to current utilizing and indicating means, as previously described. In accordance with the embodiment just described, regardless of its momentary position, the cylindrical element It} remains magnetically linked to the mu-lti-tu-rn winding 34 on the ring core 3 2.

In order to reduce mechanical strains to which the tubular element I0 is subjected by temperature variations which cause an expansion differential between element 10 and attached leads 1643, I have devised a further modification in accordance with the present invention, which is shown in FIGURE 7 of the drawings.

In this latter figure, the magnetoresistive body is folded back on itself, to form a pair of concentric cylinders, 10A and 1013, each having a thickness of, for example, 20 mils, which are joined together at one end, and which are insulated from each other by a cylindrical sleeve 45, comprising for example, glass, or ceramic or the like, having a thickness of the order of 10 mils. The electrodes 12 and 14 are applied to the adjacent free ends of the cylinders 10A and 1013, in the manner previously described with reference to FIGURES 5A and 5B. The assemblage including magnetoresistive element 10A and 10B and insulating element 45 moves into and out of the gap 22 of the magnet 27, described in detail with reference to FIGURES 5A, 5B and 6. The heavy wire leads l6 and 18 are rigidly fastened onto electrodes 12 and I4, and are fastened together to form a single primary loop which is inductively linked to the ring core 32, forming a torodial transformer coupling as previously de scribed with reference to FIGURE 6. The total length of the magnetoresistive path is now doubled, as is its resistance and the thermal expansions of the inner and outer sleeves 10A and 10B are equal thereby eliminating thermal strain.

FIGURE 8 shows the schematic diagram of the magnetoresistive element 10 coupled to the single turn primary winding of the transformer, the secondary being the multiturn Winding 34, both about the core 32. At alternat ing cur-rent of such frequency as the transformer is designed for, the input impedance Z is closely equal N R where R is the resistance of the magnetoresistive element 10, and N is the number of turns of the multiturn stationary winding. For example, for R equal to one milliohm, and N equal to 1000, and neglecting the small resistance of wiresld and 18, the impedance Z equals 0 ohms, a convenient magnitude. Z may then be measured by conventional means, such as, for example, an alternating current Wheatstone bridge circuit, not shown, or utilized for any one of the number of Well-known applications.

It will be apparent to those skilled in the art, that the teachings of the present invention are readily applicable to numerous different types of electromechanical systems.

For example, a microphone embodying the principles of the present invention is shown in schematic crosssection in FIGURE 9 of the drawings. This combination includes a tubular magnetoresistive element 10, such as shown and described with reference to FIGURE 4, mounted concentrically with, and movable with respect to the radial, magnetic field, of a coaxial permanent magnet 27, in the manner described with reference to FIG- URES 5A and 5B, like designations indicating substantially similar elements. The magnet 27 is rigidly mounted in a conventional insulated housing, which has not been shown. The magnetoresistive element 10 is soldered by means of for example, lead solder, to the inwardlydirected central portion of a conventional, metallic diaphragm 38. The periphery of the diaphragm 38 is 7 rigidly mounted on a projecting flange connected to the outer shell of the magnet 27. The magnetoresistive element 10 is supported by a conventional means which permits it to move readily with the diaphragm 3d.

Leads 16 and 13 are connected to the magnetoresistive element 10, in the manner indicated by the previous figures. A direct current source 29 of, for example, about one volt, is included in the circuit to supply current fior modulation by the variations in resistance of the element 10. Leads 16 and 18 are connected to a conventional amplifying circuit through a conventional transformer 42., which has a turns ratio designed to provide the proper impedance match to the input of the amplifier stage 47. Transistor amplification which has relatively low input impedance may be found convenient for use with a circuit of this type.

Although indium antimonide doped in the manner described in the earlier pages of the specification to have electron mobilities of the order of about 75,000 centimeters per second per volt per centimeter at room temperature may be considered as a preferred material for certain embodiments of the invention, for other embodiments in which it is desired to utilize a magnetoresistive element having a substantially greater resistivity, indium arsenide having electron mobilities of about 40,000 centimeters per second per volt per centimeter, at room temperature and resistivity nearly 100 times as great as that of indium antimonide, may be preferred. In addition, other materials such as germanium and silicon, properly doped in a manner known in the art to include a requisite number of n-type impurities, may also be used. In fact, any material may be used for the purposes of the present invention which is highly magnetoresistive. In the case of the semi-conductive materials mentioned above, this is believed to be a function, in part at least, of the electron mobilities which characterize these materials, which at room temperatures are within the range from about 1,000 to 100,000 centimeters per second per volt per centimeter.

Moreover, while polycrystalline material, being simple to obtain and to prepare, has been specified as adequate for the purposes of the present invention, it is conceivable also that single crystalline materials may be used if desired for certain types of applications, providing such materials have substantially uniform magnetoresistive characteristics along at least two crystallographic axes.

In addition to the several embodiments shown and described herein to illustrate the principles of the present invention, numerous other embodiments in accordance with the present invention will readily occur to those skilled in the art, such as, for example, modifications of the various magnetoresistive devices disclosed by Willardson and Beer, supra.

What I claim is:

l. A circuit comprising in combination a magnet provided with an annular gap with the magnetic flux passing radially therethrough, a magnetoresistive element of hollow cylindrical shape interposed in said gap and substantially concentric therewith, electrode means coupled to the two flat ends of said cylinder, means for longitudinally moving said element with respect to said gap to vary the magnetic flux linked by said element, and means including a voltage source and current detecting and utilizing means electrically coupled to said element thru said electrode means and controlled by the magnetic flux linked by said element.

2. A circuit in accordance with claim 1 wherein said magnetoresistive element is characterized by current carrier mobilities within the range of about 20,000 to 100,- 000 centimeters per second per volt per centimeter at room temperature.

3. A control circuit comprising in combination, a magnet, an element interposed in the field due to said magnet, said element having a hollow cylindrical shape radially out by the field of said magnet wherein the over-all radius of said element substantially exceeds the radial thickness of the Walls, said element comprising a material having current carrier mobilities Within the range of about 20,000 to 100,000 centimeters per second per volt per centimeter at room temperature, a pair of electrodes connected to the flat ends of said cylindrical element, a circuit comprising a voltage source and current detecting and utilizing means coupled to said electrodes, and means for longitudinally moving said element with respect to the field of said magnet for controlling said circuit.

4. A circuit comprising in combination a magnet having concentric inner and outer circular pole pieces which together form an annular gap with the magnetic flux passing radially therethrough, a magnetoresistive element of hollow cylindrical form interposed in said gap and substantially concentric therewith, a pair of annular electrodes connected to the flat annular surfaces of said element, a

transformer, said transformer having a single-turn primary Winding and a multi-turn secondary winding, the terminals of said primary winding being connected to the electrodes of said pair, and a circuit including a voltage source and current indicating and utilizing means connected to the terminals of the secondary winding of said transformer, and means for moving said magneto-resistive element parallel to the axis of said gap for controlling said circuit.

5. A control circuit in accordance with claim 4 wherein one of said pole-pieces is notched toaccommodate a portion of the said primary winding, thereby to facilitate the movement of said magnetoresistive element in said p- 6. A control circuit in accordance with claim 4 wherein said magnetoresistive element is folded back uponitself to form an annular slot, a hollow insulating cylinder being interposed in said slot, said electrodes being formed on the external annular ends of said folded cylinder, outside of said gap, wherein the terminals of said primary winding are connected to said electrodes.

7. In combination, a magnet having an annular gap, a magnetic field being imposed'in a radial direction across said gap, and a tubular element consisting essentially of magnetoresistive material, disposed concentrically in said gap and movable in the direction of the principal axis of said gap. I

A device including a hollow cylindrical element consisting essentially of magnetoresistive material, an electrical circuit including a voltage source and current detectmg and utilizing means electrically coupled to the ends of said hollow cylindrical element, magnetic means produc- 1ng a magnetic field radially through the hollow cylindrical element, and means for longitudinally moving said hollow cylindrical element with respect to said magnetic field for varying the magnetic flux concentrated in said element by the magnetic field to vary the electrical res stance of said element and control the current in said circuit.

if. A device including a hollow cylindrical magneto- ISSlStlVt) element consisting of at least two concentrically superimposed hollow cylindrical portions formed of magnetoresrstive material and longitudinally electrically arranged in series, an electrical circuit including a voltage source and current detecting and utilizing means electrically coupled to said hollow cylindrical element with said portions in series, magnetic means producing a ma"- netic field radially through the hollow cylindrical elemen t, and means for varying the magnetic flux concentrated in said element by the magnetic field to vary the electrical resistance of said element and control the current in said circuit.

10. A device including a hollow cylindrical magnetoresistive element consisting of at least two concentrically superimposed hollow cylindrical portions formed of magnetoresistive material and longitudinally electrically arranged in series, an electrical circuit including a voltage source and current detecting and utilizing means electrically coupled to said hollow cylindrical element with said portions in series, magnetic means producing a magnetic afield radially through the hollow cylindrical element, and

' and control the current in said circuit.

' References Cited in the file of this patent UNITED STATES PATENTS 2,251,900 Smith Aug.5,1941 2,536,805 Hansen Jan. 2, 1951 2,536,806 Hansen Jan. 2, 1951 10 2,553,491 Shockley May 15, 1951 2,608,837 Lecuir July 22, 1952 2,632,062 Montgomery Mar. 17, 1953 2,633,521 Becker Mar. 31, 1 953 2,736,858 We-lker Feb. 28, 1956 2,838,696 Thurston June 10, 1958 2,852,732 Weiss Sept. 16, 1958 2,945,379 Barnes July 19, 1960 OTHER Y REFERENCES 

1. A CIRCUIT COMPRISING IN COMBINATION A MAGNET PROVIDED WITH AN ANNULAR GAP WITH THE MAGNETIC FLUX PASSING RADIALLY THERETHROUGH, A MAGNETORESISTIVE ELEMENT OF HOLLOW CYLINDRICAL SHAPE INTERPOSED IN SAID GAP AND SUBSTANTIALLY CONCENTRIC THEREWITH, ELECTRODE MEANS COUPLED TO THE TWO FLAT ENDS OF SAID CYLINDER, MEANS FOR LONGITUDINALLY MOVING SAID ELEMENT WITH RESPECT TO SAID GAP TO VARY THE 